[modules] Introduce new VariableLocation for module imports/exports.

src/ast/prettyprinter.cc

Index: src/ast/prettyprinter.cc
diff --git a/src/ast/prettyprinter.cc b/src/ast/prettyprinter.cc
index 57f5bec187f143c59b39dd6e3e884f6e2f3f5ae1..83aa5339216f235dfd9e9c57aa50ae6b48c1b55c 100644
--- a/src/ast/prettyprinter.cc
+++ b/src/ast/prettyprinter.cc
@@ -1063,6 +1063,9 @@ void AstPrinter::VisitVariableProxy(VariableProxy* node) {
       case VariableLocation::LOOKUP:
         SNPrintF(buf + pos, " lookup");
         break;
+      case VariableLocation::MODULE:
+        SNPrintF(buf + pos, " module");
+        break;
     }
     PrintLiteralWithModeIndented(buf.start(), var, node->name());
   }